1. Field
Embodiments of the present invention relate to an inverter control apparatus and a control method thereof and, more particularly, to an inverter control apparatus and a control method thereof to stably operate a three-phase motor using a capacitor having a small capacitance for a DC link.
2. Description of the Related Art
FIG. 1 illustrates a waveform of a conventional q-axis current reference.
Referring to FIG. 1, a DC-link voltage waveform a1, a q-axis current reference waveform a2 in a trapezoidal form, and a q-axis current reference waveform a3 in a sine squared form are illustrated.
To stably operate a permanent magnet synchronous motor with an inverter using a small-capacitance capacitor, it is important to generate a rotor based q-axis current reference and a rotor based d-axis current reference.
Rotor based q-axis current reference may be modulated into a sine squared form and a rotor based q-axis current reference may be modulated into a trapezoidal form.
However, the method of modulating a rotor based q-axis current reference into a sine squared form does not consider a voltage drop due to inductance and stator resistance and needs an additional methodology for measuring the voltage of input power, such as a voltage sensor.
The method of modulating a rotor based q-axis current reference into a trapezoidal form cannot eliminate harmonic components of an input current.
In conventional methods, the rotor based d-axis current reference uses a voltage equation (Equation 1) and voltage limit equation (Equation 2), or a d-axis current is controlled to have a negative value in a specific region having an insufficient DC-link voltage.
                                          V            ds            r                    =                                                    R                s                            ⁢                              i                ds                r                                      +                                          L                d                            ⁢                              ⅆ                                  ⅆ                  t                                            ⁢                              i                ds                r                                      -                                          ω                r                            ⁢                              L                q                            ⁢                              i                qs                r                                                    ⁢                                  ⁢                              V            qs            r                    =                                                    R                s                            ⁢                              i                qs                r                                      +                                          L                q                            ⁢                              ⅆ                                  ⅆ                  t                                            ⁢                              i                qs                r                                      +                                          ω                r                            ⁡                              (                                                                            L                      d                                        ⁢                                          i                      ds                      r                                                        +                                      λ                    pm                                                  )                                                                        [                  Equation          ⁢                                          ⁢          1                ]                                                      V            dc                                3                          ≥                                            V              ds                              r                *                2                                      +                          V              qs                              r                *                2                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
However, when a small-capacitance capacitor may be used for a DC link of an inverter, the DC link voltage of the inverter pulsates at double a source frequency, and thus a voltage limit circle varies even in a steady state. Accordingly, instantaneous change of the d-axis current deteriorates system stability.